Sometimes, in order to meet specific requirements, it is sought to locally modify physical properties, such as the hardness, magnetism or ductility of micromechanical horological parts. This is particularly the case with the LIGA method, which is an increasingly frequently used method within the field of timepiece design. This method allows the mass production of micromechanical parts with quite remarkable precision and with an overall cost which remains compatible with industrial manufacturing requirements. However, as will be seen in detail below, the micromechanical horological parts obtained using the LIGA method sometimes have problems of wear resistance which it is sought to overcome.
The acronym “LIGA” is from German. It is an abbreviation of “Lithographie, Galvanoformung, Abformung” which represent the different steps of the method. In short, the LIGA method used within the field of horology consists in exposing a photosensitive resin deposited on a conductive substrate to UV rays through a photolithographic mask. After the resin has been developed, a galvanisation step is carried out by electroplating, during which metal is deposited in the microstructures which were developed beforehand in the layer of photosensitive resin and whose contours match the shapes of the desired parts. The last step consists in removing the remaining layer of photosensitive resin and separating the components thereby obtained.
Within the field of horology, the LIGA method has now been of interest for several years. However, as will have been understood from the foregoing, one of the constraints imposed by the LIGA method is that the material used must be able to be deposited by electrolysis. Within the field of horology, the first material used was nickel. This material has the advantage of being able to be implemented in the LIGA method. However, it has the drawback of being magnetic in the amorphous state, which makes it difficult to use for horological applications.
The Applicant is currently interested in micromechanical horological parts obtained via the LIGA method using a nickel and phosphorus alloy containing 12% by weight of phosphorus and which will be called “alloy NiP12” below. This alloy NiP12 has the great advantage of being non-magnetic in the amorphous state. However, the hardness of alloy NiP12 parts made by the LIGA method is average, on the order of 580 HV. Problems of wear directly linked to this relatively low hardness appear in certain conditions in these micromechanical horological parts.
Faced with this problem of wear, the Applicant sought to increase the hardness of the gear teeth obtained by the LIGA method. A known technique in the field of manufacturing solid mechanical parts, for example for the aeronautic or automobile industry, consists in locally heating the gear teeth. It is known, however, that although localised heating of solid parts, for example using a laser beam, has the effect of increasing the hardness of the heated zones, the heating is accompanied by a weakening of the zones thus treated. However, in the field of manufacturing solid mechanical parts, the thermal inertia of the parts is such that only the directly heated surfaces undergo a phase transformation, while the phase of most of the part keeps its intrinsic features.
However, the same is not true in the field of horological design where the parts commonly have thicknesses on the order of several tens to several hundreds of microns and where dimensions rarely exceed a millimetre.